Discussion
Intestinal macrophages are known as “gatekeepers” for their ability to clear foreign pathogens by expressing pathogen-associated molecular patterns (PAMPs) and recognize tissue damage by damage-associated molecular patterns (DAMPs). They also show tolerance to food or commensal microbes. Macrophages are a heterogeneous population of innate immune cells that play diverse functions in immune response and tissue homeostasis. Although boundaries of the heterogeneous macrophage population are ambiguous, they can be functionally divided into pro-inflammatory versus anti-inflammatory macrophages. Macrophage polarization depends on their microenvironment. Disruption of the composition or function of macrophages can contribute to various gastro-intestinal diseases such as IBD. IBD patients show an altered population of macrophages where inflammatory macrophages are dominant. These inflammatory macrophages can accumulate in the intestine during IBD, which can release TNF-α, IL-1β, and IL-6. To overcome IBD, biologics such as anti-TNF therapy is commonly used since TNF-α is a key pathological cytokine in IBD. Anti-IL-12/23, anti-integrin, 5-aminosalicylic acids, and immunomodulators can also be used for IBD therapy. Despite recent advances in IBD therapy, limitations still exist since complete remission is difficult. Thus, the goal for overcoming IBD is shifting to targeting mucosal healing.
Since macrophages are deeply involved in regulating inflammation and mucosal healing, targeting these macrophages can be a good therapeutic strategy against IBD. Many previous studies targeting macrophages to induce an anti-inflammatory shift have shown the potential of an effective IBD therapy. Activation of SIRT1 can promote anti-inflammatory genes Mrc1 and IL-10 while decreasing pro-inflammatory genes iNOS, CCL2, IL-12 p35, and IL-12 p40. Anti-inflammatory macrophage polarization by the SIRT1/adenosine monophosphate-activated protein kinase pathway can reduce inflammatory responses in a rheumatoid arthritis mouse model (S. Park et al., 2020; Zhou, Zhang, & Ding, 2022). TLR2/1 agonist PAM3CSK4 can induce monocytes to differentiate into anti-inflammatory macrophages and reduce DSS-induced colitis in a mouse model (Horuluoglu, Kayraklioglu, Tross, & Klinman, 2020).
Metabolic pathways in macrophages are deeply involved in the fate of macrophage polarization. Therefore, reports targeting macrophage polarization to an anti-inflammatory character by metabolic modulation is arising as a promising strategy for IBD therapy. Pro-inflammatory macrophages depend on glycolysis, while anti-inflammatory macrophages are activated by mitochondrial oxidative phosphorylation (Liu et al., 2021). Different paths of arginine catabolism contribute to macrophage polarization. Generation of NO by iNOS favors inflammatory macrophage polarization, while generation of urea and orthinine by Arg1 favors anti-inflammatory macrophage polarization. Increased consumption of L-arginine by iNOS and decreased Arg1 have been shown in ulcerative colitis. Thus, dietary supplementation of L-arginine can be used as an IBD therapy (Andrade et al., 2018; Bourgonje et al., 2020).
LMT503 is an NAD+ enhancer by working as an efficient substrate for NQO1. NQO1 has multiple roles in defending against cellular stress. NQO1 can detoxify quinones by reducing them into hydroquinones. At the same time, oxidation of NAD(P)H occurs, which increases NAD+. Previous studies have shown that inflammation can be controlled by triggering macrophage shift by increasing NAD+ level. NAD+synthesis via the kynurenine pathway can shift aged inflammatory macrophages to an anti-inflammatory state with increased phagocytic activity (Minhas et al., 2019). Increasing NAD+ by administration of NAD+ precursor β-nicotinamide mononucleotide (β-NMN) can shift macrophages to an anti-inflammatory type during sepsis (Cros et al., 2022). Sirtuins are a family of class III histone deacetylases known to be dependent on NAD+(Anderson, Madsen, Olsen, & Hirschey, 2017). They consist of seven members, SIRT1-7. Sirtuins can modulate inflammation via nuclear factor-kappa B (NF-κB) or activator protein 1 (AP-1). SIRTs except SIRT5 are known to inhibit NF-κB activation by deacetylation of p65 subunit. SIRT1 cam inhibit IL-1β secretion by down-regulating NLRP3 inflammasome (S. Park et al., 2020). SIRT1, SIRT3, and SIRT6 can decrease AP-1 transcriptional activity which controls pro-inflammatory cytokine production (Palomer et al., 2020; Zhang et al., 2010). Treatment of BMDMs with LMT503 increased SIRT1, 3, and 6.
In summary, this study found that NAD+ enhancement alleviated intestinal inflammation by reinforcing immune regulatory character of tissue resident CX3CR1+macrophages. NAD+ enhancement was achieved by LMT503, an efficient substrate for NQO1. Metabolic reprograming in macrophages treated with LMT503 down-regulated inflammatory markers TNF-α and iNOS but up-regulated anti-inflammatory IL-10 and Arg1 during colitis. NAD+ enhancement of colonic CX3CR1+ macrophages was crucial for ameliorating DSS-induced colitis in mice since LMT503 treatment did not show any effect when CX3CR1+macrophages were depleted. Our study suggests that LMT503 is a potential IBD therapy candidate that can drive anti-inflammatory macrophage polarization via immuno-metabolic modulation.